Vascular endothelial cells (EC) are a major target of injury in xenograft rejection and in vascular diseases. The long term objective of this laboratory is to understand mechanisms of injury to EC, especially those caused by complement activation, and to develop approaches to prevent injury. These objectives are pursued by studying xenotransplantation models in which EC injury by the membrane attack complex of complement (MAC) is of paramount importance in graft rejection. We have recently discovered that following activation, porcine EC become resistant to the injurious effects of the MAC. Porcine EC and human complement or blood are used as models for pig-to-human transplantation as the pig is considered a potential donor. We hypothesize that this inducible resistance of porcine EC to the MAC, and possibly to other mediators of damage, results from EC activation, which in turn causes impairment of the EC activation triggered by the MAC. This hypothesis will be tested using models of porcine-to-human xenotransplantation, as follows. 1. We will establish the specificity, scope and mechanisms of protection against xenogeneic injury achieved in EC by stimulation with an alphagalactosyl-specific agonist, the lectin Bandeiraea simplicifolia BS-I. These in vitro studies will determine which of the known proinflammatory effects of the MAC are abrogated in EC, in addition to the lytic action of the MAC. Experiments will also establish whether resistance to damage by NK cells, a known mechanism of xenogeneic injury, also takes place, and if resistance correlates with activation of antiapoptotic genes; the role of alphagal antigen density in the establishment of resistance; the mechanisms of altered MAC structure and function; and the impairment of the activation triggered by the MAC in lectin-treated cells. 2. Because injury to EC in a graft or in vascular diseases occurs under the dynamic influence of blood circulation, studies will be carried out with an isolated porcine kidney perfused ex vivo with human blood. The EC of the organ will be activated to resist injury by exposure to lectin and then subjected to the injurious effects of human blood. Our hypothesis will be evaluated with studies similar to those for the in vitro model. These studies will provide fundamental knowledge of mechanisms that could be manipulated to protect the vascular endothelium when a solid organ xenograft is transplanted into a human. This may potentially contribute to the success of clinical xenotransplantation to meet the need for organ replacement. This knowledge may also be of importance for innovative strategies to control vascular diseases such as arteriosclerosis and vasculitis.